1. Field of Invention
This invention relates to processes for preparing polyimides and to certain novel polyimides and more particularly to processes for preparing polyimides which are soluble in phenolic solvents.
2. Prior Art
The prior art is replete with references teaching the preparation of polyimides and polyimide precursors by the reaction of aromatic tetracarboxylic acids, or anhydrides thereof, with aromatic diamines. While the polyimide articles formed have excellent heat resistance, it is difficult to form articles because of their insolubility and intractability. This has necessitated the use of polyimide precursors which can be shaped into the desired form and then cured to the polyimides. However, a difficulty with the precursors is their release of by-products during curing which can result in powdering and formation of voids in the article or the use of polyimides having lower thermal stability. To minimize or avoid these problems, thin layers must be used, especially in wire enameling, and the desired thickness built up by applying numerous layers. Thus, there is a continuing need in the art for thermally stable polyimides that can cure in thick sections without the aforementioned difficulties.
References illustrative of the state of the art are U.S. Pat. No. 3,277,043, issued Oct. 4, 1966 to Fred F. Holub, U.S. Pat. No. 3,678,015, issued to Fred F. Holub et al. and U.S. Pat. No. 3,666,709, issued May 30, 1972 to Munchiko Suzuki et al. The former teaches a process for preparing phenolic solvent soluble polymers by the reaction of an anhydride and diamine in a phenol under initially substantially anhydrous conditions. The latter teaches a phenolic solvent soluble polyimide by reacting an aromatic tetracarboxylic acid and a diamine in a phenolic solvent at a temperature above the boiling point of water. The second patent shows a process for preparing a polyimide precursor by reacting a diamine and an aliphatically unsaturated hydrocarbon monoanhydride (with up to 10% dianhydride) in the presence of an inert hydrocarbon solvent and an inert phenolic solvent.